Numerous types of small, high-performance resonators utilizing piezoelectric materials have been put into practical use, such as the film bulk acoustic wave resonator (FBAR) and the surface acoustic wave (SAW) resonator. The film bulk acoustic wave resonator has recently drawn particular attention with regard to applications requiring high resonant frequencies of, for instance, 5 GHz and higher, because it is structurally easier to increase in resonant frequency than the surface acoustic wave resonator.
The film bulk acoustic wave resonator basically consists of an upper electrode, a lower electrode and a piezoelectric film between the two electrodes. The desired resonance characteristics can be obtained by applying high-frequency signal between the upper and lower electrodes. The resonant frequency of a film bulk acoustic wave resonator depends mainly on the thicknesses of the upper electrode, lower electrode and piezoelectric film. These thicknesses are set with reference to the wavelength determined by the ratio of the acoustic velocity of the bulk wave to the resonant frequency (acoustic velocity/resonant frequency). The acoustic velocity of the bulk wave is determined by the physical properties of the materials constituting the respective films (their elastic constants etc.), so that the wavelengths of the respective films become shorter as the resonant frequency is set higher. In other words, the thickness of the piezoelectric film and other films has to be made thinner to achieve a higher resonant frequency.
Therefore, where a high resonant frequency of 5 GHz or higher is desired, the thickness of the piezoelectric film must be made very thin. When the ZnO is used as the material of the piezoelectric film, for example, the thickness of the film must be made about 0.27 μm to realize a resonant frequency of 5 GHz or higher.
Owing to the fact that the piezoelectric film exhibits pyroelectricity, however, a potential difference is liable to arise between the upper electrode and lower electrode in the course of film bulk acoustic wave resonator fabrication. When this potential difference becomes large, electric discharge occurs between the upper and lower electrodes. Since this discharge passes through the interposed piezoelectric film, it may damage the piezoelectric film and thus lower the reliability and yield of the product. Moreover, the likelihood of such discharge increases with decreasing piezoelectric film thickness, so that it is a particularly serious problem in the case of fabricating a film bulk acoustic wave resonator having a high resonant frequency of 5 GHz or higher.